The present invention relates to methods of making organic light-emitting devices and, more particularly to methods of providing in a light-emitting layer of an organic light-emitting device formed by diffusing a dopant from a dopant layer to a dopant receiving layer.
Organic light-emitting devices, also referred to as organic electroluminescent (EL) devices or as organic internal junction light-emitting devices, contain spaced electrodes separated by an organic light-emitting structure (also referred to as an organic EL medium) which emits light in response to the application of an electrical potential difference across the electrodes. At least one of the electrodes is light-transmissive, and the organic light-emitting structure can have a multi-layer of organic thin films which provide for hole injection and transport from an anode, and for electron injection and transport from a cathode, respectively, with light emission resulting from electron-hole recombination at an internal junction formed at an interface between the hole-transporting and the electron-transporting thin films. As employed herein, the term xe2x80x9cthin filmxe2x80x9d refers to layer thicknesses of less than 1 micrometer with layer thickness of less than about 0.5 micrometer being typical. Examples of organic light-emitting devices containing organic light-emitting structures and cathode constructions formed by thin film deposition techniques are provided by commonly-assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; and 4,769,292.
During operation of an organic light-emitting device, the spectral distribution of emitted light (measured in terms of spectral radiance) is related to the electroluminescent properties of the organic thin films used in the device construction. For example, if an organic light-emitting structure includes a layer which contains a light-emitting host material, the emitted light will be dominated by the light emission from the host material.
The above-cited commonly-assigned U.S. Pat. No. 4,769,292 recognized that advantageous performance features of an organic light-emitting device could be obtained if the device included a luminescent zone (or light-emitting layer) of less than 1 micrometer in thickness and comprised of an organic host material capable of sustaining hole-electron recombination, and a small amount of fluorescent material capable of emitting light in response to energy released by hole-electron recombination. The introduction of a fluorescent material into a layer of a light-emitting host material will modify the color of the light emission, and can improve the operational stability of an organic light-emitting device. In analogy to terminology used in the semiconductor industry, fluorescent materials dispersed uniformly at relatively low concentration in light-emitting organic host materials are called xe2x80x9cdopants.xe2x80x9d
As currently practiced, the organic thin films of a light-emitting device are formed by vapor deposition (evaporation or sublimation) in successive deposition steps within a vacuum system which employs a deposition rate control. When a fluorescent dopant is to be uniformly incorporated within an organic light-emitting layer, the light-emitting host material and the fluorescent dopant material are co-deposited from two independently controlled deposition sources. It is necessary to control the individual deposition rates of a fluorescent dopant and a host material when a desired dopant concentration in the host material of the organic light-emitting layer is at or near a lower end of a dopant concentration range of 10xe2x88x923 to about 10 mole percent. The difficulty of reliably controlling the deposition rates of an organic light-emitting host material and of a fluorescent dopant material has been an obstacle in the process of reproducibly fabricating organic electroluminescent devices containing a fluorescent dopant or fluorescent dopants.
Another recent method for fabrication of electroluminescent devices disclosed in WO 99/39373 uses patterning of the components, but does not operate on a completed device, hence requiring breaking of the vacuum to pattern, followed by completion of the device under vacuum or inert atmosphere.
It is an object of the present invention to provide an improved method of forming a doped emissive layer which overcomes difficulties associated with prior art methods.
Accordingly, it is an object of the present invention to provide a method of making an electroluminescent device having a substrate, an anode provided over the substrate, and at least one dopant receiving layer which when doped provides an emissive layer disposed over the anode and a cathode disposed over the emissive layer, and diffusing the dopant form the dopant layer into the dopant receiving layer.
These objects are achieved in a method of making an electroluminescent device having a substrate, and at least one dopant receiving layer containing a host material which when doped provides an emissive layer, comprising the steps of:
(a) providing at least one dopant layer having a dopant disposed over or under the dopant receiving layer;
(b) providing an anode and a cathode so that the dopant receiving layer and the dopant layer are disposed between such anode and cathode; and
(c) heating the electroluminescent device to cause the dopant to diffuse from the dopant layer into the dopant receiving layer and forming the emissive layer having uniformly dispersed dopant in the host material.
An important feature of the invention is that the dopant layer can be patterned prior to diffusing into the dopant receiver layer, thus producing a patterned, multi-color emissive device.
Another feature of the invention is that it separates the deposition of the organic dopant receiver material forming the emissive layer from the deposition of an organic dopant material, and introduces the dopant material into the host material after completion of the processing of the device.
By separating the depositing step of an organic light-emitting layer from the depositing step of a fluorescent dopant layer, each of these layers can be formed separately to a desired thickness, thereby obviating the problems of deposition rate control associated with the prior art method of forming a doped light-emitting layer. Thus, the deposition processes are greatly simplified and require equipment of reduced complexity.
The light-emitting layer of the organic light-emitting host material can be formed by conventional vapor deposition (evaporation, sublimation) and, alternatively, by other coating of a polymeric organic light-emitting material.
The dopant layer can be formed by conventional vapor deposition and, alternatively, by thermally induced transfer from a dopant donor layer formed on a donor support or other printing methods such as ink jet, gravure, offset, screen, flexographic, or xerographic printing.
Two or more dopant layers containing different dopants can be formed in a particular pattern, thereby providing a broader range of options for modifying the light emitted from the light-emitting layer.
Diffusion of a dopant or of dopants from a dopant layer or from dopant layers into an organic light-emitting layer by a heat treatment process of the completed device requires relatively simple equipment and provides accurate control of processing to achieve uniform dispersion of the dopant or dopants throughout the organic light-emitting layer of the host material.